What subwoofer? Most require line-level signals, and won't work with speaker power levels from an amp. The power sent to a speaker is too high, and it will overdrive the electronics. But there are subwoofers out there that do accept speaker inputs, so the exact make/model matters. Generally, you'll see speaker terminals on the sub when they support speaker inputs. Even with those, don't hook speaker outputs to RCA jacks.

But even if the sub supports it, if you are pulling the signal from another speaker in the pin that you've also left connected to the pin's amp, I would say that's bad. The pin's amp can barely drive the speakers installed in the cabinet and backbox. Asking it to drive anything else seems like a bad idea.

I don't have first-hand experience with Mandalorian, but it looks like it's a Spike 2 system, which means it probably has seven pins on the C5 connector that's used for line-out. You can use pins 1 and 7 for the sub and it should work (pin 1 is the negative terminal/ground, and pin 7 is the LFE signal).

You say you've read all the posts about the topic, but the discussions do cover this specific scenario, so it's unclear what part about those posts haven't helped you. But from your description, it sounds like you aren't using the line-out header, and that's what's wrong.

If you still need help, please provide a lot more detail than you have. Include precise information about your wiring, preferably with in-focus, detailed photos, and be more specific about what "noise" you are getting. You say it's "from the pin sub", by this do you mean the larger single speaker in the cabinet? What does the noise actually sound like? A buzzing? A hum? Distortion in other audio signals? When does it occur? All the time? Just when there is other audio from the game?

FWIW, I connected an external sub to my Rush pin, using a homemade connector for the C5 header on the motherboard. I left the backbox and cabinet speakers all connected, and it works perfectly. Here's my post describing what I did, with links to parts and other posts on Pinside that I used as a reference. That post is in the main thread for DIY sound, and it is chock full of incredibly useful information.

Sorry, not sure how I missed that. I edited my previous post to include the link that should've been there the first time.

And it happens as soon as you connect the cable, or only after some other event triggers the problem (e.g. audio from the pin)?

Oddly, the Elac web site doesn't seem to have any information that DS81-BK model. But, I don't see any reason to believe that the subwoofer you're using is designed to deal with speaker-level signals. The buzz could be some kind of ground loop, but rather than try to figure out how to address the sound directly, IMHO you should change the connections so that you're using the correct output from the pin to drive the sub, i.e. the LFE output from the C5 connector on the motherboard.